Pertussigen (pertussis toxin) (Ptx) is the toxic protein from Bordetella pertussis responsible for most of the known biological activities of pertussis vaccine. We are presently studying the ability of Ptx to induce lymphocytosis in mice in order to find the type(s) of lymphocytes mainly affected by Ptx. To this end, we prepared lymphocyte suspensions from various lymph organs (thymus, lymph nodes, spleen and blood) and determined the percentage of each different type of lymphocyte by means of the fluorescence activated cell sorter (FACS). The fluoresceinated antibodies used to stain each type of lymphocyte were specific to Ig, IgM, Lyt-1, Lyt-2 and Thy 1.2 antigens of these cells. The percentage of each cell type in normal and Ptx-treated mice were determined by the FACS. Ptx decreased the percentage of T-lymphocytes (Thy 1.2 containing cells), including helper (Lyt-1+) and suppressor T-cells (Lyt-2+). In contrast, a marked increase in the percentage of B cells in the blood (Ig+) and a decrease in the percentage of these cells in the spleen and lymph nodes was observed. No changes were noticed in the percentage distribution of T cells in the thymus of Ptx treated mice. Similar changes were seen, to different degrees, in various inbred strains of mice (C57B1/10J, C57 B1/6J DBA, BALB/c, ASW, SJL, A/WysN). Nu/Nu mice (athymic nude mice) also showed a marked leukocytosis after the injection of Ptx with an increase in the percentage of IgM containing cells. Normal Nu/Nu mice contain a small percentage of Thy 1.2 + cells. The percentage of these cells was decreased by treatment with Ptx. Ptx increased markedly the percentage of IgE containing lymphocytes in the spleen and blood (only tissues studied) of many strains of mice, but it was most pronounced in C57B1/10J and C57B1/6J mice.